The invention is based on an apparatus for using geothermal energy and method of generating it.
A heat flow, flowing from the interior of the Earth, can be ascribed to an extent of about 80% to the decay of radioactive isotopes in rocks, and about 20% to the primordial heat generated in the creation of the Earth. The temperature from the Earth""s surface to the interior of the Earth increases by about 30xc2x0 C. every 1000 meters. In the vicinity of geothermal anomalies, greater temperature increases can occur, for instance because of a geographic closeness to hot magma chambers (intrusions) in the crust of the Earth, flows of deep-seated bodies of water, and so forth. Such regions are especially preferred for utilizing the geothermal energy.
The geothermal energy can be stored in various substances, fundamentally in water vapor, water, and/or hot strata of rock.
Stored water vapor is usually located in the vicinity of active volcanoes and highly fissured or water-bearing bedrock. The vapor can be carried to the Earth""s surface andxe2x80x94after an often-necessary separation of entrained waterxe2x80x94used to generate electricity. However, usable vapor sources exist in only a few regions.
The energy stored in the hot water in strata of the Earth are as a rule reached by an apparatus known as a doublet. The doublet has a feed bore, through which the hot water from the various strata of the Earth can be pumped to the Earth""s surface. To prevent the hydrostatic pressure in the strata from dropping over time, and to avoid lessening the inflow of hot water and in particular to enable better utilization of the heat of the rock, the cooled water is returned to the corresponding strata of the Earth via an injection bore. From the injection bore, the water can flow through the rock to the feed bore and is heated in the process. The water pumped out of the strata of the Earth is as a rule used for space heating.
For tapping the energy stored in hot layers of rock, a so-called hot dry rock process is known. In this process, in a first method step a first deep hole is drilled down to about 5000 meters. Via the deep hole, water is forced at high pressure to a depth of about 5000 meters into the rock, causing cracks to form in the rock. With a second deep hole, at a distance of about 500 meters, the region where the cracks have formed is drilled into. Next, water can be pumped into the rock via one deep hole and carried through the cracks to the second deep hole and then pumped, heated, to the Earth""s surface via the second deep hole. To avoid expensive dry holes, the cracks in the rock have to be located. At a depth of 5000 meters, this is especially difficult and complicated.
Another apparatus of this generic type is known that has three concentrically arranged pipes, specifically a so-called outer descending pipe, a cladding tube, and an inner riser pipe. Cold water is pumped downward into the interior of the Earth through the descending pipe, heated, and having been heated, is pumped upward through the riser pipe. An annular gap with air between the riser pipe and the cladding tube serves as an insulator between the outer descending pipe and the inner riser pipe. The pipes are surrounded on the outside, toward the soil, by a water-impermeable suspension, for instance of betonite. A mass transfer with the soil is avoided. However, the efficiency for generating electrical energy is low.
U.S. Pat. No. 4,644,750 discloses an apparatus of this generic type, in which a working substance is pumped downward into the ground via a first pipe into a deep hole and upward via capillaries. By capillary action, the attempt is made to keep external power for pumping the working substance upward from below as slight as possible
The object of the invention is to refine the apparatus of this generic type and in particular to create an apparatus and a method with which, economically and in a way that can be achieved simply from a technological standpoint, a circular process for recovering energy can be created, which is maintained without external power or with only very slight external power.
The invention is based on an apparatus for using geothermal energy, having at least a first conduit, through which a working substance is carried downward into the interior of the Earth, and having at least one second conduit, through which the working substance is carried upward, in the direction of the surface of the Earth, and the first and second conduits form a closed system relative to the soil.
It is proposed that the working substance is carried downward into the interior of the Earth, and the throttling region throttles the largely liquid working substance upstream, in the flow direction, of the throttling region to a vaporization pressure, so that downstream of the throttling region the working substance is largely completely vaporizable. The working substance can advantageously be carried downward in the liquid state and upward in the gaseous state, at lower density. The liquid pressure that builds up, or the force of gravity acting on the liquid, can be utilized for a requisite pressure elevation in a clockwise cycle process, and as a result an especially high efficiency can be achieved, especially in the generation of electrical energy. In principle, however, a pump can also be used to generate a requisite pressure upstream of the throttling region; the pump acts to reinforce the liquid column or generates the requisite pressure by itself.
Once the working substance downstream of the throttling region has vaporized extensively, and advantageously completely, it flows upward to the Earth""s surface. During operation of the apparatus, it is possible to avoid introducing an external power, in particular pumping power, which thus enhances the efficiency. A slight pressure drop is established in the second conduit, from bottom to top in the gas column. At the Earth""s surface, the working substance in vapor form has a much higher pressure than in an initial state of the process, and this can especially advantageously be used to generate electrical energy. The throttling region can be formed by one throttling point, or advantageously by a plurality of throttling points, which are disposed spaced apart from one another in the flow direction. When there are more than one throttling point, an especially high pressure can be achieved in the upper region of the second conduit.
In a further feature of the invention, it is proposed that a compressor, by way of which the working substance is compressable upstream of the throttling region, is disposed downstream in the flow direction of the second conduit. The throttling region can especially advantageously be disposed in the upper region of the first conduit, and a large temperature difference between the working substance and the interior of the Earth, and as a result a good heat transfer to from the interior of the Earth to the working substance, can advantageously be achieved over the entire first conduit. The energy delivered from the interior of the Earth can advantageously be utilized for vaporization, at a largely constant vaporization temperature of the working substance. In the second conduit, the working substance largely has the same temperature as in the first conduit, as a result of which an insulation between the conduits can be omitted. To recover a high quantity of heat, even a slight hole depth can already suffice.
The throttling region can fundamentally be formed by one or more throttling points that appear suitable to one skilled in the art. Especially advantageously, however, at least one throttling point is formed by at least one pipe. Given an adequate throttling action, a relatively low flow velocity through the throttling point and thus low wear and a long service life can be achieved.
In one feature of the invention, it is proposed that the first and second conduits communicate to form a closed system; that is, they communicate in closed fashion not only underground but also above ground. The working substance can be prevented from escaping from the system, and a working substance with which an especially high efficiency can be achieved at the lowest possible cost can advantageously be employed.
If the working substance has a lower boiling temperature than water, then in comparison to water-driven apparatuses, the working substance can already be made to vaporize at lesser depths or lower temperatures, so that drilling costs in particular can be saved. Conventional refrigerants, such as ammonia in particular, are advantageously suitable as the working substance.
To reduce the flow velocity in the first conduit and to increase the service area and thereby improve the heat transfer to the working substance, at least one element deflecting the working substance is advantageously placed at least downstream of the throttling region in the first conduit. Filler bodies, which deflect the working substance, can advantageously be introduced into the first conduit. In a further feature, it is proposed that instead of filler bodies, at least one pathway extending spirally downward is introduced, by way of which pathway the working substance is carried against the outer wall of the first conduit, as a result of which an advantageous film vaporization on the outer wall of the first conduit can be achieved. The pathway can be made from various materials; for instance, the pathway can be formed by a sheet-metal part, or it can also be embodied integrally with a conduit wall. Preferably, the pathway does not have any sealing function in the conduit.
In one feature of the invention, the first and second conduits are disposed in a deep hole that is lined imperviously, and the working substance is carried through an inner pipe downward in the radially outer region inside the deep hole and upward in the radially inner region of the deep hole. Additional deep holes can thus be dispensed with and the attendant costs eliminated. Furthermore, the conduits can be made to communicate in closed fashion underground in an especially simple, inexpensive way. In principle, however, the working substance can also be carried downward via one or more deep holes and upward via one or more other deep holes separate from the first group.
Further advantages will become apparent from the ensuing description of the drawing. The drawing shows one exemplary embodiment of the invention. The drawing, description and claims include numerous characteristics in combination. One skilled in the art will expediently assess the characteristics individually as well and put them together to make useful further combinations.